This invention relates to a method for controlling the operation of a fluidized bed reactor and, more particularly, to such a method in which the particulate size distribution of the particulate solids inventory in a circulating fluidized bed reactor is controlled.
Fluidized bed reactors are well-known and are operated by passing air through a bed of particulate material including a fossil fuel, such as coal, and an absorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at relatively low temperatures.
The most typical fluidized bed combustion utilizes what is called a "bubbling" fluidized bed in which a bed of particulate material including fuel and an adsorbent for the sulfur generated as a result of combustion of the fuel, is supported on an air distribution plate through which combustion supporting air is introduced to expand the material and cause it to take on a suspended or fluidized state.
In an effort to extend the improvements in combustion efficiency, pollutants emissions control, and operation turn down, a "circulating" fluidized bed has evolved in which relatively low fluidized bed densities are utilized when compared to those of a bubbling fluidized bed. These low densities are achieved by utilizing relatively small-sized particles and a high solids circulation. This renders the bed insensitive to fuel heat release patterns and thus minimizes the variation of the temperatures within the reactor. The resulting increase in the residence time of the adsorbent and fuel particles reduces the adsorbent and fuel consumption and results in an improved turn-down when compared to the bubbling fluidized bed.
In these circulating beds it is an advantage to be able to control the size distribution of the particulate material in the fluidized bed. Once the solid material is fed to the fluidized bed, the control of the particulate size distribution must be achieved indirectly, through a controllable system parameter that affects the particulate size distribution. Since most of the system parameters that have an effect upon the inventory of the particulate material are predetermined by systems requirements, a sufficiently flexible operating parameter must be found in the system or added to the system. The behavior of solid particles may be understood by dividing the size distribution into three groups: fine, intermediate, and coarse particles. The relatively fine particles will generally bypass the separator while the relatively coarse particles will e discharge via the bed drain. The intermediate size particles will tend to build up in the bed with the coarse particles until the size distribution of the particulate material is equal to the distribution required to drain the intermediate particles with the coarse particles. Controlling the bed size distribution would involve making the bed distribution different from the required drain material distribution. Therefore, the essential problem in controlling the particulate size distribution is to be able to dispose of both the relatively coarse and the relatively intermediate size material in such a way that the bed size distribution does not determine the size distribution of the material leaving.